It is known that the power and efficiency of an internal combustion engine can be improved by increasing the turbulence of the air fuel mixture in the combustion chamber. Many different arrangements have been proposed for this purpose. For example, spiral inlet ducts have been used to impart a swirling motion to the incoming mixture. (See, for example, U.S. Pat. Nos. 4,008,458 and 3,868,940). Shekleton U.S. Pat. No. 4,182,280 shows an arrangement of swirl inducing vanes in each of the intake ports of an internal combustion engine immediately adjacent the intake valve. Matsumota U.S. Pat. No. 4,186,706 employs a "sub-inlet system" which is arranged to introduce the fuel-air mixture into the combustion chamber in a swirling pattern at high velocity to improve flame propagation and combustion. Other swirl inducing arrangements are shown in Vogel U.S. Pat. No. 2,920,613 and List U.S. Pat. No. 3,874,357. These existing constructions are relatively costly and would be prohibitively expensive to add to existing engines. Moreover, in general, they do not provide the maximum available power, fuel economy and efficiency for the particular engine involved.